The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for chemical mechanical polishing a substrate.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a "standard" or a fixed-abrasive pad. A standard polishing pad has durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles, if a standard pad is used, is supplied to the surface of the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
One problem encountered in CMP is that a central portion of the substrate is often underpolished. This problem, which may be termed the "center slow effect", may occur even if pressure is uniformly applied to the backside of the substrate.
Another problem is the difficulty in removing the substrate from the polishing pad surface once polishing has been completed. As mentioned, a layer of slurry is supplied to the surface of the polishing pad. When the substrate is placed in contact with the polishing pad, the surface tension of the slurry generates an adhesive force which binds the substrate to the polishing pad. The adhesive force may make it difficult to remove the substrate from the pad.
Typically, the substrate is vacuum-chucked to the underside of the carrier head, and the carrier head is used to remove the substrate from the polishing pad. When the carrier head is retracted from the polishing pad, the substrate is lifted off the pad. However, if the surface tension holding the substrate on the polishing pad is greater than the vacuum-chucking force holding the substrate on the carrier head, then the substrate will remain on the polishing pad when the carrier head retracts. This may cause the substrate to fracture or chip. In addition, failure to remove the substrate can cause a machine fault requiring manual intervention. This requires shutting down the polishing apparatus, decreasing throughput. To achieve reliable operation from the polishing apparatus, the substrate removal process should be essentially flawless.
Several techniques have been employed to reduce the surface tension between the substrate and the polishing pad. Once such technique is to slide the substrate horizontally off the polishing pad to break the surface tension before vertically retracting the carrier head. This technique may, however, scratch or otherwise damage the substrate as it slides off the edge of the polishing pad. The mechanical configuration of the CMP apparatus may also prohibit use of this technique.
Another technique is to treat the surface of the polishing pad to reduce the surface tension. However, this technique is not always successful, and such treatment of the pad surface may adversely affect the finish and flatness of the substrate and reduce the polishing rate.
Another technique is to apply a downward pressure to the edge of the substrate to create a seal that prevents ambient atmosphere from interfering with the vacuum-chucking process. However, this technique may require complex pneumatic controls for the carrier head. In addition, the structure of the carrier head may prevent the application of pressure to the edge of the substrate.